<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    html,body,div {
      margin: 0;
      padding: 0;
    }
    .Site {
      display: flex;
      min-height: 100vh;
      flex-direction: column;
    }

    .Site-content {
      flex: 1;
    }
  </style>
</head>
<body class="Site">
  <header style="background: darkcyan;">header</header>
  <main class="Site-content" id="main">
    
  </main>
  <footer style="background: cyan;">footer</footer>
  <script>
    const main = document.getElementById('main');
    for (let i = 0; i < 100; i++) {
      const h1 = document.createElement('h1')
      h1.innerHTML = `${i} h1`
      main.appendChild(h1)
    }
  </script>
</body>
</html>